Examples of positional coding schemes e.g. for encoding of an audio signal are Factorial Pulse Coding (FPC) and Pyramid Vector Quantization (PVQ). A positional coding could result in large codeword indices, especially for a long input vector, because of the rapidly increasing number of combinations by increasing dimensions. For low complexity implementations, a pre-processing step of clustering of the input vector can be used. Another reason for this type of pre-processing can be quantizer restraints. However, a conventional clustering may result in that different parts of the input vector have very different sizes, which could make the positional coding very inefficient. Thus, there is a need to provide an improved pre-processing of the input vector at positional encoding.